Field Of The Invention
The present invention relates to a method of manufacturing Y--Ba--Cu--O (hereinafter referred to as YBCO) superconducting thin films having excellent crystallinity using a sputtering phenomenon. More particularly, the present invention relates to a method of optimizing film formation conditions so that the composition ratio of Y:Ba:Cu of a film becomes 1:2:3. The film is suitable for use in the electronics field of sensors, electronic devices or the like using thin films.
Description Of The Related Art
The hybrid plasma sputtering method is available as a parallel plate sputtering method for forming Y--Ba--Cu--O type superconducting thin films on substrates. The characteristic feature of the hybrid plasma sputtering method is that a high-frequency voltage is applied to the cathode electrode being superimposed on a DC voltage supplied through a low-path filter which cuts off the high-frequency voltage, wherein a conductive YBCO target is placed on the cathode and sputtered in a low vacuum. According to this method, YBCO thin films having excellent crystallinity can be obtained (see, for example, W.Ito et al., "Highest Crystallinity of a-axis YBCO films by DC-94.92 MHz Hybrid Plasma Magnetron Sputtering", Physica C 204(1993) pp.295-298).
According to the hybrid plasma sputtering method, when a bias voltage applied from a DC power source is increased at the same time as a high-frequency voltage is applied to the cathode electrode, an electric current increases sharply from a certain voltage, in correspondence with the conductance of a target, and enters into a so-called constant voltage region as shown in FIG. 7. At this time, a DC plasma is superimposed on a high-frequency plasma (hereinafter referred to as a hybrid plasma state). Use of such plasma makes it possible to speed up the film formation while maintaining the advantage of the high-frequency plasma which contributes to crystallinity. FIG. 7 shows the relationship between a bias voltage applied to the cathode and an electric current when a DC voltage and a high-frequency voltage are applied together on a YBa.sub.2 Cu.sub.3 Ox target having a composition ratio of Y:Ba:Cu of 1:2:3. The target voltage is a voltage in which a negative bias voltage generated by a DC power source is superimposed on the self bias on the cathode side generated by the high-frequency plasma which is caused by a high-frequency voltage applied to a YBa.sub.2 Cu.sub.3 Ox target. The target current is the electric current which flows when the target voltage is applied.
By optimizing the film formation conditions by using the hybrid plasma sputtering method so that the composition ratio of Y:Ba:Cu of the film becomes 1:2:3, it is possible to manufacture a YBCO thin film whose crystallinity is optimized automatically. An example of the optimized film formation conditions is as follows: the film formation pressure: 475 mTorr, the flow-rate ratio of sputtering gas Ar to reaction gas 02:2:1, the RF output: 60 W, the cathode voltage: -100 V, and the cathode current: 0.5 A.
The cathode voltage is the value of the voltage in the constant voltage region of the characteristic shown in FIG. 7. This voltage is dependent on the conductance, or the resistance of the target. Therefore, the value of the cathode voltage in the constant voltage region depends upon the target used. When, for example, a target having a low conductance is used, a voltage drop at the cathode which is necessary for generating a hybrid plasma increases.
Under the above circumstance, although the optimal film formation conditions for the composition of the film can be obtained, the film has a major problem as to its crystallinity and surface morphology (surface smoothness). The results obtained thus far reveal that, when the cathode voltage in the constant voltage region falls below -100 V by tens of volts, desired crystallinity cannot be obtained even if the composition of the film is optimized by controlling other film formation parameters.
It can be seen from this that the hybrid plasma sputtering method is a method which is sensitive to, and very restricted with regard to, the conductance of a target used. It seems that the method has a problem as a practical film formation method when the yield of the film with respect to the target is considered.